Project Summary Emotion dysregulation is recognized as an integral but insufficiently characterized influence in child psychopathology. Attention-deficit/hyperactivity disorder (ADHD) is emblematic because wide variation in both cognition and emotion dysregulation exist and are related to impairment. Consensus is emerging that effective measurement of pathophysiological mechanisms underlying cognitive and emotional features of ADHD is essential for improvement of both pharmacological and non-pharmacological treatments, but these efforts are hampered by within-group heterogeneity. Thus, understanding ADHD mechanistically will require integrating cognitive and emotional accounts of the disorder. A central puzzle concerns the joint disruption of cognition and emotion? frequently observed but poorly explained mechanistically. Two fundamental questions are addressed here: what mechanisms drive emotion dysregulation in ADHD and how can variation in emotional features be incorporated into existing nosology? The current proposal builds on prior work that suggests three temperament-based emotional profiles in children with ADHD: 1) an emotionally-normative, ?Mild? profile, 2) a positive dysregulation, ?Surgent? profile, and 3) a negative dysregulation, ?Irritable? profile. Here, that descriptive work is carried forward the next step into mechanistic and confirmatory study to advance the nosology and explanation of ADHD. Using a cross sectional, case-control design in 7-10 year-old children, Aim 1 validates and refines the proposed emotion-based phenotypes by examining their association with in vivo emotional experience using ecological momentary assessments. This is combined in Aim 2 with an experimental approach to identify reactive and regulatory mechanisms contributing to individual differences in emotion dysregulation in ADHD. Here, sequential sampling models are combined with time-locked measures of central and peripheral nervous system functioning adding novelty. Two types of low-cost, clinically-translatable measures are emphasized: eye-tracking/pupillometry and electroencephalogram-measured event-related potentials (ERPs). Finally, Aim 3 identifies biobehavioral correlates of emotion-related impairment that are directly relevant to development of novel treatments. The theoretically-driven, multi-level approach that is used directly addresses RDoC goals related to multi-method integration and defining mechanisms of complex behavior.